From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Leslie Rhorer" Subject: RE: mdadm raid1 read performance Date: Fri, 6 May 2011 16:20:39 -0500 Message-ID: <89.70.16951.22664CD4@cdptpa-omtalb.mail.rr.com> References: <20110505111006.GB11441@www2.open-std.org> Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110505111006.GB11441@www2.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: =?iso-8859-1?Q?'Keld_J=F8rn_Simonsen'?= , 'NeilBrown' Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids > -----Original Message----- > From: linux-raid-owner@vger.kernel.org [mailto:linux-raid- > owner@vger.kernel.org] On Behalf Of Keld J=F8rn Simonsen > Sent: Thursday, May 05, 2011 6:10 AM > To: NeilBrown > Cc: Liam Kurmos; Roberto Spadim; Brad Campbell; Drew; linux- > raid@vger.kernel.org > Subject: Re: mdadm raid1 read performance >=20 > On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote: > > On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos > wrote: > > > > > as a separate question, what should be the theoretical performanc= e of > raid5? > > > > x(N-1) > > > > So a 4 drive RAID5 should read at 3 time the speed of a single driv= e. >=20 > Actually, theoretically, it should be more than that for reading, mor= e > like N minus > some overhead. In a raid5 stripe of 4 disks, when reading you do not = read > the checksum block, and thus you should be able to have all 4 drives > occupied with reading real data. Some benchmarks back this up, > http://home.comcast.net/~jpiszcz/20080329-raid/ > http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.= html > The latter reports a 3.44 times performance for raid5 reads with 4 > disks, significantly over the N-1 =3D 3.0 mark. >=20 > For writing, you are correct with the N-1 formular. There have been a lot of threads here about array performance, but one important factor rarely mentioned in these threads is network performance. Of course, network performance is really outside the scop= e of this list, but I frequently see people talking about performance well i= n excess of 120MBps. That's great, but I have to wonder if their network actually can make use of such speeds. Of course, if the application actually obtaining the raw data is on the machine, then network perform= ance is much less of an issue. A database search implemented directly on th= e server, for example, can use every bit of performance available to the = local machine. Given that in my case the vast majority of data is squirted a= cross the LAN (e.g., these are mostly file servers), anything much in excess = of 120MBps is irrelevant. I mean, yeah, it=92s a rather nice feeling that= my RAID arrays can deliver more than 450MBps if they are ever called upon = to do so, but with a 1G LAN, that's not going to happen very often. I just w= onder how many people who complain of poor performance can really benefit all= that much from increased performance? -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html